1. Field of the Invention
The present invention relates to a laser light beam generating apparatus. More particularly, the present invention relates to a laser light beam generating apparatus including a light source which generates a second harmonic laser light beam using a non-linear optical crystal element.
2. Background of the invention
It has hitherto been proposed to effect efficient wavelength conversion by taking advantage of a high power density in a resonator. For example, investigations are conducted in an external resonation type second harmonic generation or second harmonic generation (SHG) by a non-linear optical element within a laser resonator.
As an example of a laser light beam generating apparatus as a solid state laser generating the second harmonics within a resonator, it has been known to provide an apparatus having a resonator including a pair of reflective mirrors, a laser medium provided between the reflective mirrors and a non-linear optical crystal element provided between one of the mirrors and the laser light medium. In this apparatus, the second harmonic laser light beam may be generated efficiently by phase-matching the second harmonic laser light beam with respect to the laser light beam having the fundamental wavelength.
For realizing the phase matching, type I or type II phase matching conditions need to be established between the laser light beam having the fundamental wavelength and the second harmonic laser light beam. The type I phase matching is based on the principle of producing a phenomenon in which a sole photon having a double frequency is created from two photons polarized in the same direction by taking advantage of the ordinary light of the laser light beam having the fundamental wavelength. On the other hand, the type II phase matching is based on the principle of causing the eigen polarization of the two perpendicular fundamental wavelengths to fall on a non-linear optical crystal element to establish phase matching conditions for each of the two eigen polarization light beams. The laser light beam of the fundamental wavelength is divided into an ordinary light beam and an extraordinary light beam within the non-linear optical crystal element. As a result, the phase matching is carried out for the ordinary light and the extraordinary light beam with the extraordinary light of the second harmonic laser light beam.
However, if it is desired to generate a second harmonic laser light beam using type II phase matching conditions, there is the risk that generation of the laser light of the second harmonics cannot be maintained stably. Because, the eigen polarization is changed in phase each time the laser light beam of the fundamental wavelength traverse the non-linear optical crystal element.
If two perpendicular eigen polarizations, that is p-and s-waves, are deviated in phase each time the laser light beam having the fundamental wavelength produced in the laser medium traverses the non-linear optical crystal element repeatedly by a resonant operation, a strong resonant state, that is strong standing wave, cannot be produced. This is because the steady-state operation in which the two eigen polarizations of the laser light beam of the fundamental wavelength strengthen each other efficiently in each part of the resonator cannot be produced. As a result, the conversion efficiency of the laser light beam of the fundamental wavelength into the laser light beam of the second harmonics is deteriorated with the risk of noise generation in the second harmonics laser light beam.
The present Assignee has already proposed in Japanese Patent KOKAI Publication No. 1-220879 (JP-A-01 220879) a laser light beam generating apparatus to resolve the above-described problem. This laser light beam generating apparatus generating a second harmonic laser light beam using a non-linear optical crystal element has a birefringent element as a quarter wave plate arranged in a resonant optical path of the laser light beam of the fundamental wavelength. In this type laser light beam generating apparatus, the second harmonic laser light beam as an output laser light beam is radiated in a stabile condition.
FIG. 1 shows an example of a laser light beam generating apparatus disclosed in the above-mentioned Japanese Patent KOKAI publication No. 1-220879 (JP-A- 01 220879). The laser light beam generating apparatus shown in FIG. 1 includes a resonator 101 which has a laser medium 102 a pair of reflecting surfaces 103, 105, a non-linear optical crystal element 106 and a birefringent element 107 as a quarter wave plate. The laser medium as a Nd:YAG is rod-shaped. One of the reflecting surfaces 103 as a dichroic mirror is formed on the incident surface of the laser medium 102. The other reflecting surface 105 is a dichroic mirror and is formed on the inside surface of an output concave mirror 104. The non-linear optical crystal element 106 is made of KTP (KTiPO.sub.4). The birefringent element 107 is made of a quartz plate. The laser medium 102, the non-linear optical crystal element 106 and the birefringent element 107 are provided between the pair of reflecting surfaces 103, 105.
The incident surface of the laser medium 102 arranged in the resonator 101 is irradiated by a pumping laser light beam emitted from a semiconductor laser 111 as a pumping light source via a collimator lens 112 and an objective lens 113. As a result, the laser medium 102 generates a laser light beam of a fundamental 20 wavelength LA(.omega.). The laser light beam of the fundamental wavelength LA(.omega.) is transmitted through the non-linear optical crystal element 106 and the birefringent element 107 and reflected back at the reflective surface 105 of the concave mirror 104 so as to be transmitted again through the birefringent element 107, the non-linear optical crystal element 106 and the laser medium 102 in this order before being reflected by the incident surface of the laser medium 102. Consequently, the laser light beam of the fundamental wavelength LA(.omega.) performs a resonating operation by being propagated back and forth between the reflective surface 103 of the .laser medium 102 and the reflective surface 105 of the concave mirror 104 of the resonator 101.
The birefringent element 107, such as the quarter wave plate, has its optical axes so set that, within a plane perpendicular to the direction of light propagation, the direction of the refractive index n.sub.e(7) of an extraordinary light is inclined by a predetermined azimuth angle .theta. shown in FIG. 2, for example .theta.=45.degree., with respect to the direction of the refractive index of the extraordinary light beam n.sub.e(6) of the non-linear optical crystal element 106.
In the above-described laser light beam generating apparatus, a laser light beam of second harmonics LA(2.omega.) is generated by the laser light beam of fundamental wavelength when the laser light beam of the fundamental wavelength is transmitted through the non-linear optical crystal element 106. The laser light beam of the second harmonics LA(2.omega.) is transmitted through the concave mirror 104 and outputted as an output laser light beam.
In this state, the ordinary light and the extraordinary light beam of the laser light of the fundamental wavelength LA(.omega.) are passed through a birefringent element 107. The birefringent element can be defined by a quarter waveplate having a wavelength equal to one-fourth of the wavelength of the laser light beam of the fundamental wavelength, which is set in an azimuth angle .theta. of 45.degree. with respect to the non-linear optical crystal element 106. As a result, the power of the laser light beam in each region of the resonator 101 is stabilized at a predetermined level. The birefringent element 107 inhibits coupling between two perpendicular intrinsic polarization modes of the laser light beam of the fundamental wavelength LA(.omega.) caused by generation of the sum frequency, when the laser light beam of fundamental wave length LA(.omega.) generated in the laser medium 102 is passed through the non-linear optical crystal element 106 in resonance for generating the type II laser light beam of the second harmonics, thereby stabilizing the oscillation.
If an optical system is employed which converges the generated SHG laser light beam on an optical conjugate point to a virtual luminance point of the resonator, for example, a point on an optical disc, for applying the above-described type II SHG laser light beam generating apparatus, a return light beam to the resonator is produced. Although it is possible to reduce the return laser light beam by an optical system using a Faraday element or a quarter wave plate and a polarization prism, it is difficult to reduce the return light beam to zero due to manufacturing tolerances of the optical components and to birefrigence produced at the optical disc.
In the case of the above-described SHG laser light beam generating apparatus only the SHG laser light beam is returned, so that there is no problem of a mode hop noise due to only a minor amount of the light beam being returned as in the case of the conventional semiconductor laser light source. However, the return light beam interferes with the light beam generated at the SHG laser light beam generating apparatus, so that interference noise is produced if the length of the light path is fluctuated on the order of a wavelength.
If the reflectivity of intensity of the return laser light beam of the SHG laser light beam from the optical disc is R.sub.d and the reflectivity of intensity of the mirror within the laser light beam generating apparatus is R.sub.m, the peak-to-peak value N.sub.pp of the interference noise is given by the formula ##EQU1##
Consequently, even if R.sub.d and R.sub.m are each 1%, the peak-to-peak value N.sub.pp produces an interference noise of 4%, which in turn deteriorates signal characteristics. An interference noise with a backward output is the same as that for R.sub.m =1, that is 100%, reflectivity so that, even if the return light beam is 1%, the peak-to-peak value N.sub.pp produces an interference noise even reaching 40%, thus tending to modify a laser output.